This application claims the priority of German patent document 197 52 773.6, filed Nov. 28, 1997, the disclosure of which is expressly incorporated by reference herein. Commonly-assigned U.S. patent application Ser. No. 09/201,134, identifying the same inventors, having the same filing date, and corresponding to the same subject matter of the present application, is referenced.
The present invention relates to a process for manufacturing an air-gap-insulated exhaust elbow of an exhaust system of a vehicle which contains an exhaust-gas-carrying inner pipe constructed as a pipe bend. At least one branched inner pipe adjoins the pipe bend. The system further includes an outer jacket as well as entry flanges for fastening the exhaust elbow to a cylinder head of an internal-combustion engine and an exit flange for coupling the exhaust elbow to the additional exhaust gas pipe systems. The inner pipe is constructed as a pipe bend, on one end, and is connected by way of a sliding fit with one end of the inner pipe of the branched inner pipe. The other end of the pipe bend and the branching of the branched inner pipe are fixedly connected with one entry flange respectively. The outer jacket is fixedly connected in the area of the end of the branched inner pipe which is free of a connection with an entry flange and or inner pipe of an exhaust pipe, being fixedly connected with the exit flange, and the outer jacket being arranged at a distance around the inner pipes while forming the air insulating gap.
DE 195 11 514 C1 describes a process for manufacturing an exhaust elbow which consists of several inner pipes, which are fitted into one another by a sliding fit, an outer jacket, entry flanges and an exit flange. The outer jacket is constructed in a half-shell design, the fitted composite of the inner pipes (pipe bend, T-piece, branching pipe with the connection to the exit flange) being placed into a lower outer jacket half shell. Then the upper half shell is pressed onto the lower half shell and is welded to the lower half shell while forming a double-flanged seam between the inner pipe ends.
The fitted composite of the inner pipes in the known process is centered within the outer jacket in a high-expenditure manner by special spacer rings which are pushed onto several inner pipes, the gap which is created in this case forming the future air insulating gap. The spacer rings consist of a material which decomposes and/or sublimes under the effect of heat, particularly during the engine operation. Since, on the one hand, the individual pipes having manufacturing tolerances can be slid with respect to one another and, because of the mounting operation, have different fitting lengths from one fitted composite to the next. On the other hand, the spacer rings themselves are subjected to manufacturing tolerances and, because of their design relative to the construction of the bottom shell, will rarely rest against it in a surrounding manner. The manufacturing of the whole exhaust elbow is subjected to tolerances even under these aspects. In the above-mentioned manufacturing tolerances, the inner pipe with the branching connection piece is virtually never situated with the desired defined surrounding air gap inside the outer jacket. A precise reproducibility does not exist here.
During the assembly, care must be taken that a certain minimal fitting length is maintained so that the individual inner pipes will not slide apart. This maintenance of the minimal fitting length requires perceptiveness and considerable expenditures. Vibrations and centrifugal forces may also occur during the transfer of parts to the welding station, which leads to another shifting of the individual inner pipes with respect to one another and with respect to the bottom shell of the outer jacket. This may even lead to a disassembly of the fitted composite.
Because of the rebound distortion of the two sheet metal half shells after the deep-drawing, the two outer jacket half shells do not continuously rest unaided against one another in a close-fitting manner and therefore without any gap. In the welding station, the top shell of the outer jacket is therefore placed onto the bottom shell and pressed against it. Vibrations will also occur here which affect the fitted composite, and there is a displacement of the relative position of the branched inner pipe in the outer jacket.
Finally, the shells of the outer jacket are laser-welded to one another. After the contact pressing is eliminated, because of the non-uniformity of the contact surfaces of the half shells, considerable tension forces are exercised on the weld seam. This reduces the continuous loadability of the assembly, particular of the outer jacket, and, in the operation of the exhaust gas system, may even lead to a failure of the component. The process reliability of the manufacturing of the exhaust elbow, on the whole, is therefore not sufficiently ensured.
The welding-together of the half shells while forming a double flanged seam also requires relatively high expenditures, particularly since, at the transition to the cutout of the outer jacket for the branching connection piece of the inner pipe, because of edge radii, a triangular wedge is formed which must be closed by welding for ensuring the reliability of the process which, in practice, can be carried out only by a filler metal. In addition, because of its design, the double flanged seam can be mechanically stressed to a limited extent. Also, for fixing the inner pipe to the outer jacket, a welded connection is required while forming a circular seam, i.e., a surrounding fillet weld seam, in the end area of the branching connection piece. The end of the inner pipe of the connection piece is slightly set back with respect to the opening of the outer jacket.
In addition, particularly because of the branched exhaust pipe, the outer pipe spatially has a very projecting construction because, when the half shells are deep drawn, no branching can be achieved. Thus, a construction of an outer jacket which is true to the contours can, therefore, not be obtained with respect to the design of the inner pipe. All inner pipes are integrally enclosed by a single common outer jacket, whereby, because of the uniform end of the outer jacket approximately in the plane of the entry flange, relatively large-volume sheet metal sections of the outer jacket are formed between the inner pipes adjoining the entry flanges which require considerable space, increase the weight of the branched exhaust pipe and result in additional unnecessary expenditures of material. As a result, the construction of a defined completely uniform air gap also cannot be achieved in the branched exhaust pipe.
Furthermore, because of the outer jacket, engines of different numbers of cylinders require exhaust elbows of different constructions. This results in high additional manufacturing and tool expenditures connected with the corresponding costs. Also, for differently designed spaces, new variants of the exhaust elbow construction in the half shell design must be thought up which are adapted to these spaces. The respective implementation also requires considerable manufacturing expenditures.
An object of the present invention to provide an improved process such that a manufacturing of space-saving air-gap insulating exhaust elbows is achieved in a simple manner which is process-reliable and can be precisely reproduced.
According to the present invention, this object has been achieved by providing that the exhaust elbow is joined together of several air-gap-insulated exhaust pipes composed of a pipe-bend-shaped and at least one branched exhaust pipe, as well as the pertaining entry flanges and the exit flange. The exhaust pipes are shaped in an air-gap-insulated manner by an internal high-pressure metal forming process for one double pipe respectively consisting of two mutually coaxially arranged pipes in an internal high-pressure metal forming tool with the introduction of a pressure fluid between the exterior wall of the inner pipe and the interior wall of the outer pipe of the double pipe forming the outer jacket. While forming a passage opening from the inner pipe toward the outside, at the end of the branching connection piece, a double-walled section is cut out. The ends of the air-gap-insulated exhaust pipes which are to be connected with one another are first trimmed while opening up the respective air insulating gap and are then fitted into one another. The ends are shaped such that the fitted connections of the outer pipes and of the inner pipes of the exhaust pipes to be connected are made with play. The mutually connected outer pipe ends of the exhaust pipes are welded to one another at the point of their fitted connection while forming a surrounding fillet weld. The connection-free ends of the exhaust pipes which are to be connected to the cylinder head are fitted together with the respective pertaining entry flanges and are welded together and the connection-free end, which faces away from the cylinder head, of one of the branched exhaust pipes is fitted together with the exit flange and is welded thereto.
The present invention permits a modular design of the exhaust elbow, in which exhaust elbows, which have an arbitrary configuration with respect to their dimension and constructional depth, can be manufactured in a simple manner from air-gap-insulated individual exhaust pipes which are fitted into one another. The outer pipes of the individual exhaust pipes are welded to one another, and the inner pipes are positioned in one another by a sliding fit.
The individual modules form the individual exhaust pipes which represent standard structural elements and therefore production goods which can be manufactured at low cost. Thus, because of the simple joining of identical parts of the branched exhaust pipes, for example, consisting of a four-cylinder exhaust elbow, a 6-cylinder or 8-cylinder exhaust elbow can be produced. The use of identical parts significantly facilitates the whole assembly.
Because of the individual exhaust pipes manufactured by internal high-pressure metal forming, any manufacturing tolerances are eliminated which result from a displacement of inner pipes situated in the fitted composite which occurs during the individual mounting and joining steps, so that each arbitrary exhaust elbow can be reproduced in a precise manner. The absence of an integral outer jacket and the fastening of the outer jacket half shells on one another as well as of the outer jacket on the entry flanges avoids the difficulties of the previously necessary welding seams resulting from mechanical thermal stress are avoided.
The construction of the outer pipe, which is true to the contours with respect to the course of the inner pipe and its shape, by way of the double pipe which is internal-high-pressure-metal-formed to an air-gap-insulated exhaust pipe, avoids superfluous material of the outer pipe in contrast to the outer jacket of the half shell construction and the space is therefore also reduced.
Overall, the construction of the exhaust elbow can be flexibly adapted to the shape of the provided constructional space because the individual exhaust pipes of the elbow can follow the course of the space because of a suitable successive joining. In contrast, the exhaust elbow in the hollow shell construction would be so voluminous because of a course of the exhaust-gas carrying pipes extending into the constructional depth that an installation is impossible from the start.
Furthermore, as the result of the manufacturing of the exhaust pipes by internal high-pressure metal forming, the air-insulating gap can be adjusted in a targeted and completely uniform manner along the whole course of the exhaust pipe. The joining points of the outer pipes on one another, while forming a surrounding, mechanically very highly stressable fillet weld, are preferably welded together by a laser.
On the whole, as a result of the manufacturing process according to the invention, a high process reliability is achieved, because of the internal high-pressure metal forming, on the one hand, there is no fitted-composite-disintegrating displacement possibility of the inner pipes and, on the other hand, the number of welds is minimized, the exhaust elbow being designed such that only surrounding, mechanically stressable fillet welds, which are easy to construct, are required for the fastening of the individual exhaust pipes to one another and to the entry flanges as well as to the xe2x80x9cexist flangexe2x80x9d.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.